Elastohydrodynamic Film Thickness Measurements of Artificially Produced Surface Dents and Grooves

Elastohydrodynamic (EHD) film thickness measurements using optical interferometry have been made of artificially produced dents and grooves under rolling and sliding conditions. These measurements are compared to stylus traces of the dent and groove profiles to determine the local deformation associated with micro-EHD pressure generation. The surface geometry associated with the dents and grooves is seen to become intimately involved in the lubrication process itself, creating local pressure variations that substantially deform the local surface geometry, particularly under sliding conditions. The rolling results have implications concerning surface initiated fatigue and the sliding results show clearly the EHD surface interactions that must occur prior to scuffing failure.     

表面形貌的微观弹流效应研究

本文在完全数值求解点接触粗糙表面弹流润滑的基础上对表面形貌的微观弹流效应和宏观弹流润滑统计结果进行分析研究,得出表面粗糙度远小于油膜厚度（膜厚比λ〉〉３）时表面形貌就已经对油膜压力存在影响。本文还就表面形貌的粗糙程度和纹理方向弹流润滑的压力和膜厚影响进行了研究,研究结果表明：表面形貌的不同将导致油膜压力的很大波动和不同,深入研究微观弹流效应有助于进一步研究润滑表面接触应力、磨损乃至零件失效的机理。     

On Mechanisms of Fatigue Life Enhancement by Surface Dents in Heavily Loaded Rolling Line Contacts

This paper studies mechanisms of surface dents in enhancing the fatigue life of rolling bearings previously reported in Akamatsu et al. (1). First, transient micro-EHL analyses of heavily loaded contacts between rough surfaces with multiple dents are conducted under near rolling conditions. Contacts with various dent dimensions, dent arrangements under different loading and kinematic conditions are investigated. Results show that surface dents generate no favorable micro-EHL effects to enhance the contact fatigue life. Subsequent analyses, in conjunction with other published studies, suggest that the fatigue life enhancement likely comes from the reduced local traction at asperity contacts through the “oil pots” effects of the dents. The effects of the surface dents on contact fatigue life may depend on the lubrication regime in which the contact is operating being favorable in poor lubrication conditions but adverse in well-lubricated contacts. Since rolling bearings are usually designed to operate in a healthy regime of lubrication, fatigue life enhancement by artificially introducing dents on bearing surfaces may not extend to field applications.     

The Influence of Moving Dent on Point EHL Contacts

A transient analysis for a dent passing through the conjunction of a point EHL contact was developed and solved numerically by using the multigrid method. Results show that the presence of sliding produces a noticeable pressure ridge and thus, a surface indentation at either leading side of the dent if the dent moves slower than the opposite surface, or at the trailing side if the dent moves faster than the opposite surface. The pressure ridge and surface indentation extend their lengths forward or backward from the dent in the sliding direction at a rate approximately half the sliding speed. The pressure fluctuation associated with the dent increases with increasing slide-to-roll ratio and dent depth, and decreases with increasing dent width in both x- and y-directions. The agreement between numerical simulation and experimental results obtained by Wedeven and Cusano (I) is remarkably close.     

表面横向粗糙峰对点接触热弹流润滑的影响

通过数值解研究丁固体表面的横向粗糙峰对钢球与玻璃盘形成的热弹流润滑接触区压力、膜厚以及温度分布的影响。计算中假设钢球是静止的．即滚滑比为2。结果表明在粗糙峰出现的位置上，压力、温度均有急剧的增加，并且固体表面的凹陷现象也变得更加复杂。     

The Influence of Tribolayer Formation on Tribological Performance of Rolling/Sliding Contacts

In the present study, a series of dedicated experiments has been performed to observe and measure the build-up of ZDDP tribolayer in rolling/sliding heavily loaded mixed-lubricated contacts. The experiments were carried out using several test configurations. First, ball-on-disc tests were run to investigate the effects of the contact pressure, temperature, and roughness on tribolayer formation. Then, micropitting tests were carried out, with bearing components and with rolling bearings, in order to evaluate the influence of tribolayer build-up on the tribological performance of the rolling/sliding contacts. The obtained experimental data were used first to calibrate a specially developed thermomechanical model and secondly to validate it under different operating conditions and at different levels, up to assessment of the tribological performance, by measuring the level of micropitting on the surfaces. The results show that a relatively simple thermomechanical model can account for tribolayer formation and removal and their effects on the contact performance in a relatively consistent way.     

In Situ Pressure and Film Thickness Measurements in Rolling/Sliding Lubricated Point Contacts

The physical conditions—pressure, temperature, shear stress—generated in an elastohydrodynamic contact govern the rheological behavior of the lubricant within the contact, and thus its tribological performance.This paper presents in situ measurements of pressure and film thickness in EHD point contacts using respectively Raman microspectrometry and differential colorimetric interferometry. A model lubricant—a polyphenylether oil (5P4E)—is used. The influence of load, temperature and sliding speed at different mean entrainment speeds on pressure and film thickness distributions is investigated. The analysis is based upon the relative values taken by the Moes dimensionless parameters M and L. In all cases, the pressure peak tends to increase and to move towards the center of the contact when the slide to roll ratio increases. When the sliding speed reaches large values (100 to 180%), several cases are encountered: When M > L, the pressure peak softens and moves towards the outlet; The Petrusevich peak disappears and the pressure profile remains almost constant as M and L are close to each other; When M < L, the peak disappears along with a strong modification of the pressure distribution. The maximum value is significantly reduced and the area of contact increases.Measured film thickness profiles confirm these tendencies. Increasing the slide to roll ratio leads to an important film thickness reduction and modifies the position of the constriction. Furthermore, film increase localized between the contact center and the constriction region is observed. This indicates the presence of important thermal effects within the contact and is discussed in the light of recently published papers.     

Measurements of Temperature Distributions around Longitudinally Grooved Rough Surfaces in Sliding Elastohydrodynamic Point Contacts

This paper describes the temperature measurements in the EHL conjunction area comprising a longitudinally grooved steel ball and a sapphire disk under high slip conditions. The authors measured the temperatures of the oil film as well as both the disk and ball surfaces; furthermore, they estimated the temperature profile across the oil film by means of experimental values. The experimental results show that the temperature of the grooved ball surface increased considerably compared with that of a non-grooved ball. The temperatures of the faster surface for the grooved ball became sensitive to the slip ratio, whereas that for the non-grooved surface was almost constant. The temperature distribution had a higher value at the land zones and a lower one at the grooved zones. The temperature rise in the grooved zones varied qualitatively depending on the thermal conditions of both the sliding surfaces.     

粗糙金属表面接触性能研究

计算粗糙金属表面接触性能时，通常忽略凸体之间的相互影响和应力场、特别是热应力场的影响，这给高副接触性能的计算带来误差。本文在考虑微凸体之间相互影响的基础上，计算了线接触金属摩擦副接触表层中的压力场、温度场及应力场（包括等温应力和热应力场）。结果表明：表面粗糙度对压力分布、温度分布及应力分布均有重要影响；热应力使最大总Ｍｉｓｅｓ应力移向表面，且随βｔ值的增大，总Ｍｉｓｅｓ应力迅速上升。这对研究胶合失效机理和边界膜成膜机理具有重要意义。     

In Situ Quantification of Oil Film Formation and Breakdown in EHD Contacts

Abstract

A measurement method using electrical impedance has been developed for simultaneous quantifications of the thickness (h) and breakdown ratio (α) of oil films in elastohydrodynamic (EHD) contacts. First, based on simplified geometrical and electrical models of EHD contacts, theoretical expressions of h and α were derived as explicit functions of the measured electrical impedance by using the Lambert function. Then, to verify the proposed measurement principle, oil film thickness measurements were conducted by using the electrical method together with the optical interferometry method in a ball-on-disc-type apparatus, which utilized the lubricated contact between a steel ball and a glass disc with a transparent conductive layer (i.e., an indium tin oxide layer). As a result, it was confirmed that the measured h-values obtained by the electrical method agreed well with those obtained by the optical method, under various test conditions with changing the entrainment speed, slide-to-roll ratio, normal load, and viscosity. Besides, it was also confirmed that the measured α-values obtained by the electrical method showed consistent correlations with the film parameter and the friction coefficient. It is hoped that the developed electrical method will be applied to practical metal-to-metal contacts (e.g., the contacts in practical ball bearings) to understand invisible behaviors of oil films in EHD contacts.     

表面织构凹坑对弹流润滑点接触摩擦力和成膜能力影响研究

零件之间形成的润滑接触在工程中广泛存在,在零件表面人工织构特定的形貌,改善接触的摩擦磨损性能,是目前工程表面设计的热点和前沿。建立等温条件下的润滑点接触分析模型,并考虑润滑粘度和密度随压力变化以及接触弹性变形,研究了表面圆柱形凹坑的直径、间距和排列方向对弹流润滑接触摩擦系数的影响。结果表明,过大或过小的凹坑直径都不利于形成油膜,降低摩擦;凹坑间距越小,对油膜形成有利,摩擦系数就越低。     

Nanoscale Reciprocating Sliding Contacts of Textured Surfaces:Influence of Structure Parameters and Indentation Depth

Textured surfaces are widely used in engineering components as they can improve tribological properties of sliding contacts, while the detailed behaviors of nanoscale reciprocating sliding contacts of textured surfaces are still lack of study. By using multiscale method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. The influence of indentation depth, texture shape, texture spacing, and tip radius on the average friction forces and the running-in stages is studied. The results show that the lowest indentation depth can make all the four textured surfaces reach steady state. Surfaces with right-angled trapezoid textures on the right side are better for reducing the running-in stage, and surfaces with right-angled trapezoid textures on the left side are better to reduce wear. Compared with other textured surfaces, the total average friction forces can be reduced by 82.94%–91.49% for the case of the contact between the tip with radius R = 60r_0 and the isosceles trapezoid textured surface. Besides,the total average friction forces increase with the tip radii due to that bigger tip will induce higher contact areas. This research proposes a detailed study on nanoscale reciprocating sliding contacts of textured surfaces, to contribute to design textured surfaces, reduce friction and wear.     

轴承沟道表面完整性研究进展

改善沟道工作表面完整性是解决轴承可靠性寿命低、摩擦功耗高、振动噪声不稳定等技术难题的有效手段。在综合调研国内外学者对广义表面完整性研究的基础上,提出了轴承沟道表面完整性特征参数的定义和内涵,阐述了特征参数的意义及对轴承性能影响的机理,简述了轴承沟道表面加工技术的发展现状,详细分析了锻造、车削、热处理、磨削、超精加工等不同加工工艺对轴承沟道表面特征参数的影响规律,给出了相应的控制措施。最后,介绍了提高轴承工作表面完整性的新技术、新工艺、新方法,并对提高轴承沟道工作表面完整性技术进行了总结和展望。     

The Tribological Performance of Black Oxide Coating in Rolling/Sliding Contacts

Different aspects of the tribological performance of black oxidized surfaces in rolling–sliding boundary-lubricated or mixed lubricated contacts are overviewed in the present work. It covers the topics of antimicropitting resistance, antismearing effects, reduction in boundary friction, repelling tribochemical attacks of aggressive lubricants or additives, running-in behavior, and the effect of black oxide coating on bearing life. The results of the component laboratory experiments, full bearing tests, and theoretical modeling, performed within the frame of the present work, shed light upon the benefits and limitations of using black oxide coating for bearing applications.     

Traction of Lubricated Rolling Contacts between Thin-Film Coatings and Steel

Tribological thin-film coatings can enhance the performance of mechanical components such as bearings and gears. Although a lubricant is present in most applications, the interactions of the lubricant with the coated surfaces are not always well understood. In the present study, Stribeck curves (i.e., traction coefficient vs. dimensionless film thickness λ) were generated for lubricated rolling contact between coated and uncoated surfaces. Chromium nitride, tungsten carbide–reinforced amorphous hydrocarbon, and silicon-incorporated diamond-like carbon coatings were evaluated. Compositions, hydrogen concentrations, Raman spectra, and surface energies are reported for the films. A ball-on-flat test configuration was used in 5%, 50%, and 100% slide-to-roll conditions. The test lubricant was a polyalphaolefin containing rust and oxidation inhibitor additives only. Differences in traction performance were observed for different coating types. Traction coefficients decreased at high λ with increased hydrocarbon content in the coating. Coating micro-texture and composition were believed to influence traction as λ became small.     

Stress Analysis on Layered Materials in Point Elastohydrodynamic-Lubricated Contacts

For multilayered or coated substrates in elastohydrodynamic-lubricated (EHL) contacts, the subsurface stress distributions under a normal load combined with shear traction have been analyzed in this article through computer simulations. The Papkovich-Neuber potentials and Fourier transform are adopted to deduce the pressure–displacement, pressure–stress, and shear traction–stress response functions in frequency domain for the coated substrates, and to calculate distributions of pressure and subsurface stress. The results from the analysis of EHL contacts on coated substrates are compared with those from dry contact model in which shear traction is assumed to obey Coulomb’s law. Effects of the Young’s modulus of coatings, the properties of lubricants, and the magnitude of traction are discussed. Similar to the results in dry contacts, hard coatings in lubricated cases tend to increase the von Mises stress, whereas soft coatings decrease the stress. Shear traction makes the max von Mises stress increasing and moving closer to surface. However, the changes in subsurface stress due to shear traction are less obvious in lubricated contacts. Comparison between EHL and dry contact models reveals that lubrication can reduce the von Mises stress in the coating layer due to smaller shear traction. The analyses show that pressure, film thickness, and subsurface stress distributions are influenced by surface coatings, sliding velocity, rheological models, and pressure–viscosity behaviors.     

Effects of Friction on the Contact and Deformation Behavior in Sliding Asperity Contacts

Finite-element analyses are carried out to study the effects of friction on the contact and deformation behavior of sliding asperity contacts. In the analysis, on elastic-perfectly-plastic asperity is brought in contact with a rigid flat at a given normal approach. Two critical values of the normal approach are used to describe the asperity deformation. One is the approach corresponding to the point of initial plastic yielding, and the other at the point of full plastic flow. Additional variables used to characterize the deformation behavior include the shape and size of the plastic zone and the asperity contact size, pressure, and load capacity. Results from the finite-element analysis show that the two values of critical normal approach decrease significantly as the friction in the contact increases, particularly the approach that causes plastic flow of the asperity. The size of the plastically deformed zone is reduced by the friction when the contact becomes fully plastic. The reduction is very considerable with a high friction coefficient, and the plastic deformation is largely confined to a small thin surface layer. For a low friction coefficient, the contact size, pressure and load capacity of the asperity are not very sensitive to the friction coefficient. For a moderate friction coefficient, the contact pressure is reduced and the junction size increased; the load capacity of the asperity is not significantly affected due to the compensating effects of the pressure reduction and the junction growth. For a high friction coefficient, the pressure-junction compensation is not longer sufficient and the asperity load capacity is reduced. The degree of the friction effects on these contact variables depends on the applied force or the normal approach. Although the analyses are conducted using a line-contact model, the authors believe that the effects of friction in sliding asperity contacts of three-dimensional geometry are essentially the same and the same conclusions would have been reached. These results may provide some guidance to the modeling of rough surfaces in boundary lubrication, in which the asperity friction coefficient can be high and vary significantly both in time and from one micro-contact to another.     

表面粗糙对乏油条件下非牛顿点接触热薄膜润滑的影响

假设运动表面为光滑表面,静止表面上有一个垂直于卷吸速度方向的横向划痕,采用Ree-Eying本构关系求解表面单一粗糙对纯滑动点接触热薄膜润滑的影响,分析处于接触区中心的表面划痕在不同的乏油程度下对油膜压力、膜厚及温度的影响。结果表明:在乏油条件下,处于静止表面上接触中心的横向划痕前后的压力和温度都有尖锐的升高,且这种升高幅度随乏油程度的严重而增加;在乏油条件下,表面粗糙的存在更不利于润滑,易于造成润滑失效。